Ugh, I hate the way Condition affects things in the game. It's ridiculous to assume that by firing a few clips continuously from the same weapon that the weapon will BREAK and that over time the bullets will deal less damage. The best aspect of the Condition system was that the lower your condition, the more likely your reload animations would slow down, because that's what wear and tear WOULD affect. Maintaining gun condition keeps it performing more efficiently, not more potently. If a rifle has fired so many rounds that the rifling has worn down and that the bullets no long spin when exiting the barrel, and that the barrel width has grown so much that the bullets might exit at potentially 90 degree angles (which is possible), that's not a gradual thing that affects bullet damage over time, that's sudden and abrupt, gun crippling damage. You retire a weapon that has such damage done to it.
If Condition had merely been the modern FO answer to the Jinxed Trait, in that the worse condition your weapon is in, the more likely you will be to experience REALLY BAD scenarios such as your clip falling out of your weapon, losing your ammo, backfiring, gun jamming, etc, then I would have loved it. As it is, it was absurd.
But that's all an aside. Yes, depending on the caliber of bullets used, shots to the head are NOT always lethal. Simple as that. Larger bullets, however, will carry so much kinetic energy that they will both "mix" the fluid brain matter while penetrating the skull, as well as leaving the skull with an explosive force that propels it opposite of the bullet's trajectory (see JFK assassination for an example). But smaller bullets that merely rely on the immediate damage of direct brain injury are quite survivable.